How to control Diatoms (Brown Algae)

Diatoms in planted tanks

What are diatoms? and more information about brown algae.

Diatoms belong to the algal class Bacillariophyceae and can be identified by yellow green or brown patches, or stringy brown masses. They are ubiquitous in virtually every freshwater and saltwater body, making up 25% of the world’s biomass. Diatoms carry out photosynthesis in the oceans, producing more than a quarter of the world’s oxygen. These algae can also be found on the surfaces of plant leaves and substrates, making them a common occurrence in aquariums regardless of the tank’s cleanliness.

Under suitable conditions, diatoms multiply rapidly, doubling in just 24 hours. However, individual cells have a short lifespan of usually less than a week. Diatoms build cell walls out of silica, leading some to speculate that sand or soil silicates are responsible for their blooms. Yet, research has demonstrated that freshwater systems are already rich in silicates, and removing them entirely is not a practical approach to controlling diatoms. Most tap water and substrates such as sand or aquasoil contain silicates.

Diatoms are especially prevalent in newly set-up planted aquariums in the first few weeks. Fortunately, they often disappear on their own without any alterations to silicate levels. As the plants settle in and the tank becomes biologically mature over three to four weeks, diatoms tend to fade. Factors such as tank maturity and plant dominance play a more significant role in preventing visible diatoms than specific water parameters or silicate levels.

New tanks are more susceptible to diatom outbreaks. Ammonia or metabolites from stressed, newly planted plants, and elevated phosphorous levels have been proposed as possible triggers. Some have found success with ion-exchange resins that absorb these triggers, but the results are inconsistent. Diatoms reproduce quickly, and many algicides are not effective against them. Proper plant husbandry and acclimation of new plants to the tank can have a significant impact on controlling algae.

Causes and Solutions


  • Biologically immature planted tanks
  • Tank instabilities that reset or disrupt the biological cycle, often caused by exotic substrates, especially for DIY dirt substrate users
  • Poor quality tap water
  • Elevated ammonia or phosphorous levels in a biologically immature tank


  • Actions to speed up tank cycling
  • Plant husbandry techniques to help plants adapt more quickly
  • Manual siphoning for mild cases
  • Use of ion-exchange resin to remove ammonia and phosphorous in desperate cases.

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